Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1367 insertions, 0 deletions

diff --git a/mozglue/baseprofiler/core/ProfileBufferEntry.cpp b/mozglue/baseprofiler/core/ProfileBufferEntry.cpp
new file mode 100644
index 0000000000..faa3826171
--- /dev/null
+++ b/mozglue/baseprofiler/core/ProfileBufferEntry.cpp
@@ -0,0 +1,1367 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ProfileBufferEntry.h"
+
+#include <ostream>
+#include <type_traits>
+
+#include "mozilla/Logging.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/Sprintf.h"
+#include "mozilla/StackWalk.h"
+
+#include "BaseProfiler.h"
+#include "mozilla/BaseProfilerMarkers.h"
+#include "platform.h"
+#include "ProfileBuffer.h"
+#include "ProfilerBacktrace.h"
+
+namespace mozilla {
+namespace baseprofiler {
+
+////////////////////////////////////////////////////////////////////////
+// BEGIN ProfileBufferEntry
+
+ProfileBufferEntry::ProfileBufferEntry()
+    : mKind(Kind::INVALID), mStorage{0, 0, 0, 0, 0, 0, 0, 0} {}
+
+// aString must be a static string.
+ProfileBufferEntry::ProfileBufferEntry(Kind aKind, const char* aString)
+    : mKind(aKind) {
+  memcpy(mStorage, &aString, sizeof(aString));
+}
+
+ProfileBufferEntry::ProfileBufferEntry(Kind aKind, char aChars[kNumChars])
+    : mKind(aKind) {
+  memcpy(mStorage, aChars, kNumChars);
+}
+
+ProfileBufferEntry::ProfileBufferEntry(Kind aKind, void* aPtr) : mKind(aKind) {
+  memcpy(mStorage, &aPtr, sizeof(aPtr));
+}
+
+ProfileBufferEntry::ProfileBufferEntry(Kind aKind, double aDouble)
+    : mKind(aKind) {
+  memcpy(mStorage, &aDouble, sizeof(aDouble));
+}
+
+ProfileBufferEntry::ProfileBufferEntry(Kind aKind, int aInt) : mKind(aKind) {
+  memcpy(mStorage, &aInt, sizeof(aInt));
+}
+
+ProfileBufferEntry::ProfileBufferEntry(Kind aKind, int64_t aInt64)
+    : mKind(aKind) {
+  memcpy(mStorage, &aInt64, sizeof(aInt64));
+}
+
+ProfileBufferEntry::ProfileBufferEntry(Kind aKind, uint64_t aUint64)
+    : mKind(aKind) {
+  memcpy(mStorage, &aUint64, sizeof(aUint64));
+}
+
+ProfileBufferEntry::ProfileBufferEntry(Kind aKind,
+                                       BaseProfilerThreadId aThreadId)
+    : mKind(aKind) {
+  static_assert(std::is_trivially_copyable_v<BaseProfilerThreadId>);
+  static_assert(sizeof(aThreadId) <= sizeof(mStorage));
+  memcpy(mStorage, &aThreadId, sizeof(aThreadId));
+}
+
+const char* ProfileBufferEntry::GetString() const {
+  const char* result;
+  memcpy(&result, mStorage, sizeof(result));
+  return result;
+}
+
+void* ProfileBufferEntry::GetPtr() const {
+  void* result;
+  memcpy(&result, mStorage, sizeof(result));
+  return result;
+}
+
+double ProfileBufferEntry::GetDouble() const {
+  double result;
+  memcpy(&result, mStorage, sizeof(result));
+  return result;
+}
+
+int ProfileBufferEntry::GetInt() const {
+  int result;
+  memcpy(&result, mStorage, sizeof(result));
+  return result;
+}
+
+int64_t ProfileBufferEntry::GetInt64() const {
+  int64_t result;
+  memcpy(&result, mStorage, sizeof(result));
+  return result;
+}
+
+uint64_t ProfileBufferEntry::GetUint64() const {
+  uint64_t result;
+  memcpy(&result, mStorage, sizeof(result));
+  return result;
+}
+
+BaseProfilerThreadId ProfileBufferEntry::GetThreadId() const {
+  BaseProfilerThreadId result;
+  static_assert(std::is_trivially_copyable_v<BaseProfilerThreadId>);
+  memcpy(&result, mStorage, sizeof(result));
+  return result;
+}
+
+void ProfileBufferEntry::CopyCharsInto(char (&aOutArray)[kNumChars]) const {
+  memcpy(aOutArray, mStorage, kNumChars);
+}
+
+// END ProfileBufferEntry
+////////////////////////////////////////////////////////////////////////
+
+// As mentioned in ProfileBufferEntry.h, the JSON format contains many
+// arrays whose elements are laid out according to various schemas to help
+// de-duplication. This RAII class helps write these arrays by keeping track of
+// the last non-null element written and adding the appropriate number of null
+// elements when writing new non-null elements. It also automatically opens and
+// closes an array element on the given JSON writer.
+//
+// You grant the AutoArraySchemaWriter exclusive access to the JSONWriter and
+// the UniqueJSONStrings objects for the lifetime of AutoArraySchemaWriter. Do
+// not access them independently while the AutoArraySchemaWriter is alive.
+// If you need to add complex objects, call FreeFormElement(), which will give
+// you temporary access to the writer.
+//
+// Example usage:
+//
+//     // Define the schema of elements in this type of array: [FOO, BAR, BAZ]
+//     enum Schema : uint32_t {
+//       FOO = 0,
+//       BAR = 1,
+//       BAZ = 2
+//     };
+//
+//     AutoArraySchemaWriter writer(someJsonWriter, someUniqueStrings);
+//     if (shouldWriteFoo) {
+//       writer.IntElement(FOO, getFoo());
+//     }
+//     ... etc ...
+//
+//     The elements need to be added in-order.
+class MOZ_RAII AutoArraySchemaWriter {
+ public:
+  explicit AutoArraySchemaWriter(SpliceableJSONWriter& aWriter)
+      : mJSONWriter(aWriter), mNextFreeIndex(0) {
+    mJSONWriter.StartArrayElement();
+  }
+
+  ~AutoArraySchemaWriter() { mJSONWriter.EndArray(); }
+
+  template <typename T>
+  void IntElement(uint32_t aIndex, T aValue) {
+    static_assert(!std::is_same_v<T, uint64_t>,
+                  "Narrowing uint64 -> int64 conversion not allowed");
+    FillUpTo(aIndex);
+    mJSONWriter.IntElement(static_cast<int64_t>(aValue));
+  }
+
+  void DoubleElement(uint32_t aIndex, double aValue) {
+    FillUpTo(aIndex);
+    mJSONWriter.DoubleElement(aValue);
+  }
+
+  void TimeMsElement(uint32_t aIndex, double aTime_ms) {
+    FillUpTo(aIndex);
+    mJSONWriter.TimeDoubleMsElement(aTime_ms);
+  }
+
+  void BoolElement(uint32_t aIndex, bool aValue) {
+    FillUpTo(aIndex);
+    mJSONWriter.BoolElement(aValue);
+  }
+
+ protected:
+  SpliceableJSONWriter& Writer() { return mJSONWriter; }
+
+  void FillUpTo(uint32_t aIndex) {
+    MOZ_ASSERT(aIndex >= mNextFreeIndex);
+    mJSONWriter.NullElements(aIndex - mNextFreeIndex);
+    mNextFreeIndex = aIndex + 1;
+  }
+
+ private:
+  SpliceableJSONWriter& mJSONWriter;
+  uint32_t mNextFreeIndex;
+};
+
+// Same as AutoArraySchemaWriter, but this can also write strings (output as
+// indexes into the table of unique strings).
+class MOZ_RAII AutoArraySchemaWithStringsWriter : public AutoArraySchemaWriter {
+ public:
+  AutoArraySchemaWithStringsWriter(SpliceableJSONWriter& aWriter,
+                                   UniqueJSONStrings& aStrings)
+      : AutoArraySchemaWriter(aWriter), mStrings(aStrings) {}
+
+  void StringElement(uint32_t aIndex, const Span<const char>& aValue) {
+    FillUpTo(aIndex);
+    mStrings.WriteElement(Writer(), aValue);
+  }
+
+ private:
+  UniqueJSONStrings& mStrings;
+};
+
+UniqueStacks::StackKey UniqueStacks::BeginStack(const FrameKey& aFrame) {
+  return StackKey(GetOrAddFrameIndex(aFrame));
+}
+
+UniqueStacks::StackKey UniqueStacks::AppendFrame(const StackKey& aStack,
+                                                 const FrameKey& aFrame) {
+  return StackKey(aStack, GetOrAddStackIndex(aStack),
+                  GetOrAddFrameIndex(aFrame));
+}
+
+bool UniqueStacks::FrameKey::NormalFrameData::operator==(
+    const NormalFrameData& aOther) const {
+  return mLocation == aOther.mLocation &&
+         mRelevantForJS == aOther.mRelevantForJS &&
+         mInnerWindowID == aOther.mInnerWindowID && mLine == aOther.mLine &&
+         mColumn == aOther.mColumn && mCategoryPair == aOther.mCategoryPair;
+}
+
+UniqueStacks::UniqueStacks()
+    : mUniqueStrings(MakeUnique<UniqueJSONStrings>(
+          FailureLatchInfallibleSource::Singleton())),
+      mFrameTableWriter(FailureLatchInfallibleSource::Singleton()),
+      mStackTableWriter(FailureLatchInfallibleSource::Singleton()) {
+  mFrameTableWriter.StartBareList();
+  mStackTableWriter.StartBareList();
+}
+
+uint32_t UniqueStacks::GetOrAddStackIndex(const StackKey& aStack) {
+  uint32_t count = mStackToIndexMap.count();
+  auto entry = mStackToIndexMap.lookupForAdd(aStack);
+  if (entry) {
+    MOZ_ASSERT(entry->value() < count);
+    return entry->value();
+  }
+
+  MOZ_RELEASE_ASSERT(mStackToIndexMap.add(entry, aStack, count));
+  StreamStack(aStack);
+  return count;
+}
+
+uint32_t UniqueStacks::GetOrAddFrameIndex(const FrameKey& aFrame) {
+  uint32_t count = mFrameToIndexMap.count();
+  auto entry = mFrameToIndexMap.lookupForAdd(aFrame);
+  if (entry) {
+    MOZ_ASSERT(entry->value() < count);
+    return entry->value();
+  }
+
+  MOZ_RELEASE_ASSERT(mFrameToIndexMap.add(entry, aFrame, count));
+  StreamNonJITFrame(aFrame);
+  return count;
+}
+
+void UniqueStacks::SpliceFrameTableElements(SpliceableJSONWriter& aWriter) {
+  mFrameTableWriter.EndBareList();
+  aWriter.TakeAndSplice(mFrameTableWriter.TakeChunkedWriteFunc());
+}
+
+void UniqueStacks::SpliceStackTableElements(SpliceableJSONWriter& aWriter) {
+  mStackTableWriter.EndBareList();
+  aWriter.TakeAndSplice(mStackTableWriter.TakeChunkedWriteFunc());
+}
+
+void UniqueStacks::StreamStack(const StackKey& aStack) {
+  enum Schema : uint32_t { PREFIX = 0, FRAME = 1 };
+
+  AutoArraySchemaWriter writer(mStackTableWriter);
+  if (aStack.mPrefixStackIndex.isSome()) {
+    writer.IntElement(PREFIX, *aStack.mPrefixStackIndex);
+  }
+  writer.IntElement(FRAME, aStack.mFrameIndex);
+}
+
+void UniqueStacks::StreamNonJITFrame(const FrameKey& aFrame) {
+  using NormalFrameData = FrameKey::NormalFrameData;
+
+  enum Schema : uint32_t {
+    LOCATION = 0,
+    RELEVANT_FOR_JS = 1,
+    INNER_WINDOW_ID = 2,
+    IMPLEMENTATION = 3,
+    LINE = 4,
+    COLUMN = 5,
+    CATEGORY = 6,
+    SUBCATEGORY = 7
+  };
+
+  AutoArraySchemaWithStringsWriter writer(mFrameTableWriter, *mUniqueStrings);
+
+  const NormalFrameData& data = aFrame.mData.as<NormalFrameData>();
+  writer.StringElement(LOCATION, data.mLocation);
+  writer.BoolElement(RELEVANT_FOR_JS, data.mRelevantForJS);
+
+  // It's okay to convert uint64_t to double here because DOM always creates IDs
+  // that are convertible to double.
+  writer.DoubleElement(INNER_WINDOW_ID, data.mInnerWindowID);
+
+  if (data.mLine.isSome()) {
+    writer.IntElement(LINE, *data.mLine);
+  }
+  if (data.mColumn.isSome()) {
+    writer.IntElement(COLUMN, *data.mColumn);
+  }
+  if (data.mCategoryPair.isSome()) {
+    const ProfilingCategoryPairInfo& info =
+        GetProfilingCategoryPairInfo(*data.mCategoryPair);
+    writer.IntElement(CATEGORY, uint32_t(info.mCategory));
+    writer.IntElement(SUBCATEGORY, info.mSubcategoryIndex);
+  }
+}
+
+struct ProfileSample {
+  uint32_t mStack;
+  double mTime;
+  Maybe<double> mResponsiveness;
+};
+
+static void WriteSample(SpliceableJSONWriter& aWriter,
+                        const ProfileSample& aSample) {
+  enum Schema : uint32_t {
+    STACK = 0,
+    TIME = 1,
+    EVENT_DELAY = 2,
+  };
+
+  AutoArraySchemaWriter writer(aWriter);
+
+  writer.IntElement(STACK, aSample.mStack);
+
+  writer.TimeMsElement(TIME, aSample.mTime);
+
+  if (aSample.mResponsiveness.isSome()) {
+    writer.DoubleElement(EVENT_DELAY, *aSample.mResponsiveness);
+  }
+}
+
+class EntryGetter {
+ public:
+  explicit EntryGetter(ProfileChunkedBuffer::Reader& aReader,
+                       uint64_t aInitialReadPos = 0)
+      : mBlockIt(
+            aReader.At(ProfileBufferBlockIndex::CreateFromProfileBufferIndex(
+                aInitialReadPos))),
+        mBlockItEnd(aReader.end()) {
+    if (!ReadLegacyOrEnd()) {
+      // Find and read the next non-legacy entry.
+      Next();
+    }
+  }
+
+  bool Has() const { return mBlockIt != mBlockItEnd; }
+
+  const ProfileBufferEntry& Get() const {
+    MOZ_ASSERT(Has(), "Caller should have checked `Has()` before `Get()`");
+    return mEntry;
+  }
+
+  void Next() {
+    MOZ_ASSERT(Has(), "Caller should have checked `Has()` before `Next()`");
+    for (;;) {
+      ++mBlockIt;
+      if (ReadLegacyOrEnd()) {
+        // Either we're at the end, or we could read a legacy entry -> Done.
+        break;
+      }
+      // Otherwise loop around until we hit the end or a legacy entry.
+    }
+  }
+
+  ProfileBufferBlockIndex CurBlockIndex() const {
+    return mBlockIt.CurrentBlockIndex();
+  }
+
+  uint64_t CurPos() const {
+    return CurBlockIndex().ConvertToProfileBufferIndex();
+  }
+
+ private:
+  // Try to read the entry at the current `mBlockIt` position.
+  // * If we're at the end of the buffer, just return `true`.
+  // * If there is a "legacy" entry (containing a real `ProfileBufferEntry`),
+  //   read it into `mEntry`, and return `true` as well.
+  // * Otherwise the entry contains a "modern" type that cannot be read into
+  // `mEntry`, return `false` (so `EntryGetter` can skip to another entry).
+  bool ReadLegacyOrEnd() {
+    if (!Has()) {
+      return true;
+    }
+    // Read the entry "kind", which is always at the start of all entries.
+    ProfileBufferEntryReader aER = *mBlockIt;
+    auto type = static_cast<ProfileBufferEntry::Kind>(
+        aER.ReadObject<ProfileBufferEntry::KindUnderlyingType>());
+    MOZ_ASSERT(static_cast<ProfileBufferEntry::KindUnderlyingType>(type) <
+               static_cast<ProfileBufferEntry::KindUnderlyingType>(
+                   ProfileBufferEntry::Kind::MODERN_LIMIT));
+    if (type >= ProfileBufferEntry::Kind::LEGACY_LIMIT) {
+      aER.SetRemainingBytes(0);
+      return false;
+    }
+    // Here, we have a legacy item, we need to read it from the start.
+    // Because the above `ReadObject` moved the reader, we ned to reset it to
+    // the start of the entry before reading the whole entry.
+    aER = *mBlockIt;
+    aER.ReadBytes(&mEntry, aER.RemainingBytes());
+    return true;
+  }
+
+  ProfileBufferEntry mEntry;
+  ProfileChunkedBuffer::BlockIterator mBlockIt;
+  const ProfileChunkedBuffer::BlockIterator mBlockItEnd;
+};
+
+// The following grammar shows legal sequences of profile buffer entries.
+// The sequences beginning with a ThreadId entry are known as "samples".
+//
+// (
+//   ( /* Samples */
+//     ThreadId
+//     Time
+//     ( NativeLeafAddr
+//     | Label FrameFlags? DynamicStringFragment* LineNumber? CategoryPair?
+//     | JitReturnAddr
+//     )+
+//     Responsiveness?
+//   )
+//   | MarkerData
+//   | ( /* Counters */
+//       CounterId
+//       Time
+//       (
+//         CounterKey
+//         Count
+//         Number?
+//       )*
+//     )
+//   | CollectionStart
+//   | CollectionEnd
+//   | Pause
+//   | Resume
+//   | ( ProfilerOverheadTime /* Sampling start timestamp */
+//       ProfilerOverheadDuration /* Lock acquisition */
+//       ProfilerOverheadDuration /* Expired data cleaning */
+//       ProfilerOverheadDuration /* Counters */
+//       ProfilerOverheadDuration /* Threads */
+//     )
+// )*
+//
+// The most complicated part is the stack entry sequence that begins with
+// Label. Here are some examples.
+//
+// - ProfilingStack frames without a dynamic string:
+//
+//     Label("js::RunScript")
+//     CategoryPair(ProfilingCategoryPair::JS)
+//
+//     Label("XREMain::XRE_main")
+//     LineNumber(4660)
+//     CategoryPair(ProfilingCategoryPair::OTHER)
+//
+//     Label("ElementRestyler::ComputeStyleChangeFor")
+//     LineNumber(3003)
+//     CategoryPair(ProfilingCategoryPair::CSS)
+//
+// - ProfilingStack frames with a dynamic string:
+//
+//     Label("nsObserverService::NotifyObservers")
+//     FrameFlags(uint64_t(ProfilingStackFrame::Flags::IS_LABEL_FRAME))
+//     DynamicStringFragment("domwindo")
+//     DynamicStringFragment("wopened")
+//     LineNumber(291)
+//     CategoryPair(ProfilingCategoryPair::OTHER)
+//
+//     Label("")
+//     FrameFlags(uint64_t(ProfilingStackFrame::Flags::IS_JS_FRAME))
+//     DynamicStringFragment("closeWin")
+//     DynamicStringFragment("dow (chr")
+//     DynamicStringFragment("ome://gl")
+//     DynamicStringFragment("obal/con")
+//     DynamicStringFragment("tent/glo")
+//     DynamicStringFragment("balOverl")
+//     DynamicStringFragment("ay.js:5)")
+//     DynamicStringFragment("")          # this string holds the closing '\0'
+//     LineNumber(25)
+//     CategoryPair(ProfilingCategoryPair::JS)
+//
+//     Label("")
+//     FrameFlags(uint64_t(ProfilingStackFrame::Flags::IS_JS_FRAME))
+//     DynamicStringFragment("bound (s")
+//     DynamicStringFragment("elf-host")
+//     DynamicStringFragment("ed:914)")
+//     LineNumber(945)
+//     CategoryPair(ProfilingCategoryPair::JS)
+//
+// - A profiling stack frame with an overly long dynamic string:
+//
+//     Label("")
+//     FrameFlags(uint64_t(ProfilingStackFrame::Flags::IS_LABEL_FRAME))
+//     DynamicStringFragment("(too lon")
+//     DynamicStringFragment("g)")
+//     LineNumber(100)
+//     CategoryPair(ProfilingCategoryPair::NETWORK)
+//
+// - A wasm JIT frame:
+//
+//     Label("")
+//     FrameFlags(uint64_t(0))
+//     DynamicStringFragment("wasm-fun")
+//     DynamicStringFragment("ction[87")
+//     DynamicStringFragment("36] (blo")
+//     DynamicStringFragment("b:http:/")
+//     DynamicStringFragment("/webasse")
+//     DynamicStringFragment("mbly.org")
+//     DynamicStringFragment("/3dc5759")
+//     DynamicStringFragment("4-ce58-4")
+//     DynamicStringFragment("626-975b")
+//     DynamicStringFragment("-08ad116")
+//     DynamicStringFragment("30bc1:38")
+//     DynamicStringFragment("29856)")
+//
+// - A JS frame in a synchronous sample:
+//
+//     Label("")
+//     FrameFlags(uint64_t(ProfilingStackFrame::Flags::IS_LABEL_FRAME))
+//     DynamicStringFragment("u (https")
+//     DynamicStringFragment("://perf-")
+//     DynamicStringFragment("html.io/")
+//     DynamicStringFragment("ac0da204")
+//     DynamicStringFragment("aaa44d75")
+//     DynamicStringFragment("a800.bun")
+//     DynamicStringFragment("dle.js:2")
+//     DynamicStringFragment("5)")
+
+// Because this is a format entirely internal to the Profiler, any parsing
+// error indicates a bug in the ProfileBuffer writing or the parser itself,
+// or possibly flaky hardware.
+#define ERROR_AND_CONTINUE(msg)                            \
+  {                                                        \
+    fprintf(stderr, "ProfileBuffer parse error: %s", msg); \
+    MOZ_ASSERT(false, msg);                                \
+    continue;                                              \
+  }
+
+BaseProfilerThreadId ProfileBuffer::StreamSamplesToJSON(
+    SpliceableJSONWriter& aWriter, BaseProfilerThreadId aThreadId,
+    double aSinceTime, UniqueStacks& aUniqueStacks) const {
+  UniquePtr<char[]> dynStrBuf = MakeUnique<char[]>(kMaxFrameKeyLength);
+
+  return mEntries.Read([&](ProfileChunkedBuffer::Reader* aReader) {
+    MOZ_ASSERT(aReader,
+               "ProfileChunkedBuffer cannot be out-of-session when sampler is "
+               "running");
+
+    BaseProfilerThreadId processedThreadId;
+
+    EntryGetter e(*aReader);
+
+    for (;;) {
+      // This block skips entries until we find the start of the next sample.
+      // This is useful in three situations.
+      //
+      // - The circular buffer overwrites old entries, so when we start parsing
+      //   we might be in the middle of a sample, and we must skip forward to
+      //   the start of the next sample.
+      //
+      // - We skip samples that don't have an appropriate ThreadId or Time.
+      //
+      // - We skip range Pause, Resume, CollectionStart, Counter and
+      //   CollectionEnd entries between samples.
+      while (e.Has()) {
+        if (e.Get().IsThreadId()) {
+          break;
+        }
+        e.Next();
+      }
+
+      if (!e.Has()) {
+        break;
+      }
+
+      // Due to the skip_to_next_sample block above, if we have an entry here it
+      // must be a ThreadId entry.
+      MOZ_ASSERT(e.Get().IsThreadId());
+
+      BaseProfilerThreadId threadId = e.Get().GetThreadId();
+      e.Next();
+
+      // Ignore samples that are for the wrong thread.
+      if (threadId != aThreadId && aThreadId.IsSpecified()) {
+        continue;
+      }
+
+      MOZ_ASSERT(
+          aThreadId.IsSpecified() || !processedThreadId.IsSpecified(),
+          "Unspecified aThreadId should only be used with 1-sample buffer");
+
+      ProfileSample sample;
+
+      if (e.Has() && e.Get().IsTime()) {
+        sample.mTime = e.Get().GetDouble();
+        e.Next();
+
+        // Ignore samples that are too old.
+        if (sample.mTime < aSinceTime) {
+          continue;
+        }
+      } else {
+        ERROR_AND_CONTINUE("expected a Time entry");
+      }
+
+      UniqueStacks::StackKey stack =
+          aUniqueStacks.BeginStack(UniqueStacks::FrameKey("(root)"));
+
+      int numFrames = 0;
+      while (e.Has()) {
+        if (e.Get().IsNativeLeafAddr()) {
+          numFrames++;
+
+          void* pc = e.Get().GetPtr();
+          e.Next();
+
+          static const uint32_t BUF_SIZE = 256;
+          char buf[BUF_SIZE];
+
+          // Bug 753041: We need a double cast here to tell GCC that we don't
+          // want to sign extend 32-bit addresses starting with 0xFXXXXXX.
+          unsigned long long pcULL = (unsigned long long)(uintptr_t)pc;
+          SprintfLiteral(buf, "0x%llx", pcULL);
+
+          // If the "MOZ_PROFILER_SYMBOLICATE" env-var is set, we add a local
+          // symbolication description to the PC address. This is off by
+          // default, and mainly intended for local development.
+          static const bool preSymbolicate = []() {
+            const char* symbolicate = getenv("MOZ_PROFILER_SYMBOLICATE");
+            return symbolicate && symbolicate[0] != '\0';
+          }();
+          if (preSymbolicate) {
+            MozCodeAddressDetails details;
+            if (MozDescribeCodeAddress(pc, &details)) {
+              // Replace \0 terminator with space.
+              const uint32_t pcLen = strlen(buf);
+              buf[pcLen] = ' ';
+              // Add description after space. Note: Using a frame number of 0,
+              // as using `numFrames` wouldn't help here, and would prevent
+              // combining same function calls that happen at different depths.
+              // TODO: Remove unsightly "#00: " if too annoying. :-)
+              MozFormatCodeAddressDetails(
+                  buf + pcLen + 1, BUF_SIZE - (pcLen + 1), 0, pc, &details);
+            }
+          }
+
+          stack = aUniqueStacks.AppendFrame(stack, UniqueStacks::FrameKey(buf));
+
+        } else if (e.Get().IsLabel()) {
+          numFrames++;
+
+          const char* label = e.Get().GetString();
+          e.Next();
+
+          using FrameFlags = ProfilingStackFrame::Flags;
+          uint32_t frameFlags = 0;
+          if (e.Has() && e.Get().IsFrameFlags()) {
+            frameFlags = uint32_t(e.Get().GetUint64());
+            e.Next();
+          }
+
+          bool relevantForJS =
+              frameFlags & uint32_t(FrameFlags::RELEVANT_FOR_JS);
+
+          // Copy potential dynamic string fragments into dynStrBuf, so that
+          // dynStrBuf will then contain the entire dynamic string.
+          size_t i = 0;
+          dynStrBuf[0] = '\0';
+          while (e.Has()) {
+            if (e.Get().IsDynamicStringFragment()) {
+              char chars[ProfileBufferEntry::kNumChars];
+              e.Get().CopyCharsInto(chars);
+              for (char c : chars) {
+                if (i < kMaxFrameKeyLength) {
+                  dynStrBuf[i] = c;
+                  i++;
+                }
+              }
+              e.Next();
+            } else {
+              break;
+            }
+          }
+          dynStrBuf[kMaxFrameKeyLength - 1] = '\0';
+          bool hasDynamicString = (i != 0);
+
+          std::string frameLabel;
+          if (label[0] != '\0' && hasDynamicString) {
+            if (frameFlags & uint32_t(FrameFlags::STRING_TEMPLATE_METHOD)) {
+              frameLabel += label;
+              frameLabel += '.';
+              frameLabel += dynStrBuf.get();
+            } else if (frameFlags &
+                       uint32_t(FrameFlags::STRING_TEMPLATE_GETTER)) {
+              frameLabel += "get ";
+              frameLabel += label;
+              frameLabel += '.';
+              frameLabel += dynStrBuf.get();
+            } else if (frameFlags &
+                       uint32_t(FrameFlags::STRING_TEMPLATE_SETTER)) {
+              frameLabel += "set ";
+              frameLabel += label;
+              frameLabel += '.';
+              frameLabel += dynStrBuf.get();
+            } else {
+              frameLabel += label;
+              frameLabel += ' ';
+              frameLabel += dynStrBuf.get();
+            }
+          } else if (hasDynamicString) {
+            frameLabel += dynStrBuf.get();
+          } else {
+            frameLabel += label;
+          }
+
+          uint64_t innerWindowID = 0;
+          if (e.Has() && e.Get().IsInnerWindowID()) {
+            innerWindowID = uint64_t(e.Get().GetUint64());
+            e.Next();
+          }
+
+          Maybe<unsigned> line;
+          if (e.Has() && e.Get().IsLineNumber()) {
+            line = Some(unsigned(e.Get().GetInt()));
+            e.Next();
+          }
+
+          Maybe<unsigned> column;
+          if (e.Has() && e.Get().IsColumnNumber()) {
+            column = Some(unsigned(e.Get().GetInt()));
+            e.Next();
+          }
+
+          Maybe<ProfilingCategoryPair> categoryPair;
+          if (e.Has() && e.Get().IsCategoryPair()) {
+            categoryPair =
+                Some(ProfilingCategoryPair(uint32_t(e.Get().GetInt())));
+            e.Next();
+          }
+
+          stack = aUniqueStacks.AppendFrame(
+              stack, UniqueStacks::FrameKey(std::move(frameLabel),
+                                            relevantForJS, innerWindowID, line,
+                                            column, categoryPair));
+
+        } else {
+          break;
+        }
+      }
+
+      if (numFrames == 0) {
+        // It is possible to have empty stacks if native stackwalking is
+        // disabled. Skip samples with empty stacks. (See Bug 1497985).
+        // Thus, don't use ERROR_AND_CONTINUE, but just continue.
+        continue;
+      }
+
+      sample.mStack = aUniqueStacks.GetOrAddStackIndex(stack);
+
+      if (e.Has() && e.Get().IsResponsiveness()) {
+        sample.mResponsiveness = Some(e.Get().GetDouble());
+        e.Next();
+      }
+
+      WriteSample(aWriter, sample);
+
+      processedThreadId = threadId;
+    }
+
+    return processedThreadId;
+  });
+}
+
+void ProfileBuffer::StreamMarkersToJSON(SpliceableJSONWriter& aWriter,
+                                        BaseProfilerThreadId aThreadId,
+                                        const TimeStamp& aProcessStartTime,
+                                        double aSinceTime,
+                                        UniqueStacks& aUniqueStacks) const {
+  mEntries.ReadEach([&](ProfileBufferEntryReader& aER) {
+    auto type = static_cast<ProfileBufferEntry::Kind>(
+        aER.ReadObject<ProfileBufferEntry::KindUnderlyingType>());
+    MOZ_ASSERT(static_cast<ProfileBufferEntry::KindUnderlyingType>(type) <
+               static_cast<ProfileBufferEntry::KindUnderlyingType>(
+                   ProfileBufferEntry::Kind::MODERN_LIMIT));
+    if (type == ProfileBufferEntry::Kind::Marker) {
+      ::mozilla::base_profiler_markers_detail::DeserializeAfterKindAndStream(
+          aER,
+          [&](const BaseProfilerThreadId& aMarkerThreadId) {
+            return (aMarkerThreadId == aThreadId) ? &aWriter : nullptr;
+          },
+          [&](ProfileChunkedBuffer& aChunkedBuffer) {
+            ProfilerBacktrace backtrace("", &aChunkedBuffer);
+            backtrace.StreamJSON(aWriter, TimeStamp::ProcessCreation(),
+                                 aUniqueStacks);
+          },
+          // We don't have Rust markers in the mozglue.
+          [&](mozilla::base_profiler_markers_detail::Streaming::
+                  DeserializerTag) {
+            MOZ_ASSERT_UNREACHABLE("No Rust markers in mozglue.");
+          });
+    } else {
+      // The entry was not a marker, we need to skip to the end.
+      aER.SetRemainingBytes(0);
+    }
+  });
+}
+
+void ProfileBuffer::StreamProfilerOverheadToJSON(
+    SpliceableJSONWriter& aWriter, const TimeStamp& aProcessStartTime,
+    double aSinceTime) const {
+  mEntries.Read([&](ProfileChunkedBuffer::Reader* aReader) {
+    MOZ_ASSERT(aReader,
+               "ProfileChunkedBuffer cannot be out-of-session when sampler is "
+               "running");
+
+    EntryGetter e(*aReader);
+
+    enum Schema : uint32_t {
+      TIME = 0,
+      LOCKING = 1,
+      MARKER_CLEANING = 2,
+      COUNTERS = 3,
+      THREADS = 4
+    };
+
+    aWriter.StartObjectProperty("profilerOverhead");
+    aWriter.StartObjectProperty("samples");
+    // Stream all sampling overhead data. We skip other entries, because we
+    // process them in StreamSamplesToJSON()/etc.
+    {
+      JSONSchemaWriter schema(aWriter);
+      schema.WriteField("time");
+      schema.WriteField("locking");
+      schema.WriteField("expiredMarkerCleaning");
+      schema.WriteField("counters");
+      schema.WriteField("threads");
+    }
+
+    aWriter.StartArrayProperty("data");
+    double firstTime = 0.0;
+    double lastTime = 0.0;
+    ProfilerStats intervals, overheads, lockings, cleanings, counters, threads;
+    while (e.Has()) {
+      // valid sequence: ProfilerOverheadTime, ProfilerOverheadDuration * 4
+      if (e.Get().IsProfilerOverheadTime()) {
+        double time = e.Get().GetDouble();
+        if (time >= aSinceTime) {
+          e.Next();
+          if (!e.Has() || !e.Get().IsProfilerOverheadDuration()) {
+            ERROR_AND_CONTINUE(
+                "expected a ProfilerOverheadDuration entry after "
+                "ProfilerOverheadTime");
+          }
+          double locking = e.Get().GetDouble();
+          e.Next();
+          if (!e.Has() || !e.Get().IsProfilerOverheadDuration()) {
+            ERROR_AND_CONTINUE(
+                "expected a ProfilerOverheadDuration entry after "
+                "ProfilerOverheadTime,ProfilerOverheadDuration");
+          }
+          double cleaning = e.Get().GetDouble();
+          e.Next();
+          if (!e.Has() || !e.Get().IsProfilerOverheadDuration()) {
+            ERROR_AND_CONTINUE(
+                "expected a ProfilerOverheadDuration entry after "
+                "ProfilerOverheadTime,ProfilerOverheadDuration*2");
+          }
+          double counter = e.Get().GetDouble();
+          e.Next();
+          if (!e.Has() || !e.Get().IsProfilerOverheadDuration()) {
+            ERROR_AND_CONTINUE(
+                "expected a ProfilerOverheadDuration entry after "
+                "ProfilerOverheadTime,ProfilerOverheadDuration*3");
+          }
+          double thread = e.Get().GetDouble();
+
+          if (firstTime == 0.0) {
+            firstTime = time;
+          } else {
+            // Note that we'll have 1 fewer interval than other numbers (because
+            // we need both ends of an interval to know its duration). The final
+            // difference should be insignificant over the expected many
+            // thousands of iterations.
+            intervals.Count(time - lastTime);
+          }
+          lastTime = time;
+          overheads.Count(locking + cleaning + counter + thread);
+          lockings.Count(locking);
+          cleanings.Count(cleaning);
+          counters.Count(counter);
+          threads.Count(thread);
+
+          AutoArraySchemaWriter writer(aWriter);
+          writer.TimeMsElement(TIME, time);
+          writer.DoubleElement(LOCKING, locking);
+          writer.DoubleElement(MARKER_CLEANING, cleaning);
+          writer.DoubleElement(COUNTERS, counter);
+          writer.DoubleElement(THREADS, thread);
+        }
+      }
+      e.Next();
+    }
+    aWriter.EndArray();   // data
+    aWriter.EndObject();  // samples
+
+    // Only output statistics if there is at least one full interval (and
+    // therefore at least two samplings.)
+    if (intervals.n > 0) {
+      aWriter.StartObjectProperty("statistics");
+      aWriter.DoubleProperty("profiledDuration", lastTime - firstTime);
+      aWriter.IntProperty("samplingCount", overheads.n);
+      aWriter.DoubleProperty("overheadDurations", overheads.sum);
+      aWriter.DoubleProperty("overheadPercentage",
+                             overheads.sum / (lastTime - firstTime));
+#define PROFILER_STATS(name, var)                           \
+  aWriter.DoubleProperty("mean" name, (var).sum / (var).n); \
+  aWriter.DoubleProperty("min" name, (var).min);            \
+  aWriter.DoubleProperty("max" name, (var).max);
+      PROFILER_STATS("Interval", intervals);
+      PROFILER_STATS("Overhead", overheads);
+      PROFILER_STATS("Lockings", lockings);
+      PROFILER_STATS("Cleaning", cleanings);
+      PROFILER_STATS("Counter", counters);
+      PROFILER_STATS("Thread", threads);
+#undef PROFILER_STATS
+      aWriter.EndObject();  // statistics
+    }
+    aWriter.EndObject();  // profilerOverhead
+  });
+}
+
+struct CounterKeyedSample {
+  double mTime;
+  uint64_t mNumber;
+  int64_t mCount;
+};
+
+using CounterKeyedSamples = Vector<CounterKeyedSample>;
+
+using CounterMap = HashMap<uint64_t, CounterKeyedSamples>;
+
+// HashMap lookup, if not found, a default value is inserted.
+// Returns reference to (existing or new) value inside the HashMap.
+template <typename HashM, typename Key>
+static auto& LookupOrAdd(HashM& aMap, Key&& aKey) {
+  auto addPtr = aMap.lookupForAdd(aKey);
+  if (!addPtr) {
+    MOZ_RELEASE_ASSERT(aMap.add(addPtr, std::forward<Key>(aKey),
+                                typename HashM::Entry::ValueType{}));
+    MOZ_ASSERT(!!addPtr);
+  }
+  return addPtr->value();
+}
+
+void ProfileBuffer::StreamCountersToJSON(SpliceableJSONWriter& aWriter,
+                                         const TimeStamp& aProcessStartTime,
+                                         double aSinceTime) const {
+  // Because this is a format entirely internal to the Profiler, any parsing
+  // error indicates a bug in the ProfileBuffer writing or the parser itself,
+  // or possibly flaky hardware.
+
+  mEntries.Read([&](ProfileChunkedBuffer::Reader* aReader) {
+    MOZ_ASSERT(aReader,
+               "ProfileChunkedBuffer cannot be out-of-session when sampler is "
+               "running");
+
+    EntryGetter e(*aReader);
+
+    enum Schema : uint32_t { TIME = 0, COUNT = 1, NUMBER = 2 };
+
+    // Stream all counters. We skip other entries, because we process them in
+    // StreamSamplesToJSON()/etc.
+    //
+    // Valid sequence in the buffer:
+    // CounterID
+    // Time
+    // ( CounterKey Count Number? )*
+    //
+    // And the JSON (example):
+    // "counters": {
+    //  "name": "malloc",
+    //  "category": "Memory",
+    //  "description": "Amount of allocated memory",
+    //  "sample_groups": {
+    //   "id": 0,
+    //   "samples": {
+    //    "schema": {"time": 0, "number": 1, "count": 2},
+    //    "data": [
+    //     [
+    //      16117.033968000002,
+    //      2446216,
+    //      6801320
+    //     ],
+    //     [
+    //      16118.037638,
+    //      2446216,
+    //      6801320
+    //     ],
+    //    ],
+    //   }
+    //  }
+    // },
+
+    // Build the map of counters and populate it
+    HashMap<void*, CounterMap> counters;
+
+    while (e.Has()) {
+      // skip all non-Counters, including if we start in the middle of a counter
+      if (e.Get().IsCounterId()) {
+        void* id = e.Get().GetPtr();
+        CounterMap& counter = LookupOrAdd(counters, id);
+        e.Next();
+        if (!e.Has() || !e.Get().IsTime()) {
+          ERROR_AND_CONTINUE("expected a Time entry");
+        }
+        double time = e.Get().GetDouble();
+        e.Next();
+        if (time >= aSinceTime) {
+          while (e.Has() && e.Get().IsCounterKey()) {
+            uint64_t key = e.Get().GetUint64();
+            CounterKeyedSamples& data = LookupOrAdd(counter, key);
+            e.Next();
+            if (!e.Has() || !e.Get().IsCount()) {
+              ERROR_AND_CONTINUE("expected a Count entry");
+            }
+            int64_t count = e.Get().GetUint64();
+            e.Next();
+            uint64_t number;
+            if (!e.Has() || !e.Get().IsNumber()) {
+              number = 0;
+            } else {
+              number = e.Get().GetInt64();
+              e.Next();
+            }
+            CounterKeyedSample sample = {time, number, count};
+            MOZ_RELEASE_ASSERT(data.append(sample));
+          }
+        } else {
+          // skip counter sample - only need to skip the initial counter
+          // id, then let the loop at the top skip the rest
+        }
+      } else {
+        e.Next();
+      }
+    }
+    // we have a map of a map of counter entries; dump them to JSON
+    if (counters.count() == 0) {
+      return;
+    }
+
+    aWriter.StartArrayProperty("counters");
+    for (auto iter = counters.iter(); !iter.done(); iter.next()) {
+      CounterMap& counter = iter.get().value();
+      const BaseProfilerCount* base_counter =
+          static_cast<const BaseProfilerCount*>(iter.get().key());
+
+      aWriter.Start();
+      aWriter.StringProperty("name", MakeStringSpan(base_counter->mLabel));
+      aWriter.StringProperty("category",
+                             MakeStringSpan(base_counter->mCategory));
+      aWriter.StringProperty("description",
+                             MakeStringSpan(base_counter->mDescription));
+
+      aWriter.StartArrayProperty("sample_groups");
+      for (auto counter_iter = counter.iter(); !counter_iter.done();
+           counter_iter.next()) {
+        CounterKeyedSamples& samples = counter_iter.get().value();
+        uint64_t key = counter_iter.get().key();
+
+        size_t size = samples.length();
+        if (size == 0) {
+          continue;
+        }
+
+        bool hasNumber = false;
+        for (size_t i = 0; i < size; i++) {
+          if (samples[i].mNumber != 0) {
+            hasNumber = true;
+            break;
+          }
+        }
+
+        aWriter.StartObjectElement();
+        {
+          aWriter.IntProperty("id", static_cast<int64_t>(key));
+          aWriter.StartObjectProperty("samples");
+          {
+            JSONSchemaWriter schema(aWriter);
+            schema.WriteField("time");
+            schema.WriteField("count");
+            if (hasNumber) {
+              schema.WriteField("number");
+            }
+          }
+
+          aWriter.StartArrayProperty("data");
+          uint64_t previousNumber = 0;
+          int64_t previousCount = 0;
+          for (size_t i = 0; i < size; i++) {
+            // Encode as deltas, and only encode if different than the last
+            // sample
+            if (i == 0 || samples[i].mNumber != previousNumber ||
+                samples[i].mCount != previousCount) {
+              MOZ_ASSERT(i == 0 || samples[i].mTime >= samples[i - 1].mTime);
+              MOZ_ASSERT(samples[i].mNumber >= previousNumber);
+              MOZ_ASSERT(samples[i].mNumber - previousNumber <=
+                         uint64_t(std::numeric_limits<int64_t>::max()));
+
+              AutoArraySchemaWriter writer(aWriter);
+              writer.TimeMsElement(TIME, samples[i].mTime);
+              writer.IntElement(COUNT, samples[i].mCount - previousCount);
+              if (hasNumber) {
+                writer.IntElement(
+                    NUMBER,
+                    static_cast<int64_t>(samples[i].mNumber - previousNumber));
+              }
+              previousNumber = samples[i].mNumber;
+              previousCount = samples[i].mCount;
+            }
+          }
+          aWriter.EndArray();   // data
+          aWriter.EndObject();  // samples
+        }
+        aWriter.EndObject();  // sample_groups item
+      }
+      aWriter.EndArray();  // sample groups
+      aWriter.End();       // for each counter
+    }
+    aWriter.EndArray();  // counters
+  });
+}
+
+#undef ERROR_AND_CONTINUE
+
+static void AddPausedRange(SpliceableJSONWriter& aWriter, const char* aReason,
+                           const Maybe<double>& aStartTime,
+                           const Maybe<double>& aEndTime) {
+  aWriter.Start();
+  if (aStartTime) {
+    aWriter.TimeDoubleMsProperty("startTime", *aStartTime);
+  } else {
+    aWriter.NullProperty("startTime");
+  }
+  if (aEndTime) {
+    aWriter.TimeDoubleMsProperty("endTime", *aEndTime);
+  } else {
+    aWriter.NullProperty("endTime");
+  }
+  aWriter.StringProperty("reason", MakeStringSpan(aReason));
+  aWriter.End();
+}
+
+void ProfileBuffer::StreamPausedRangesToJSON(SpliceableJSONWriter& aWriter,
+                                             double aSinceTime) const {
+  mEntries.Read([&](ProfileChunkedBuffer::Reader* aReader) {
+    MOZ_ASSERT(aReader,
+               "ProfileChunkedBuffer cannot be out-of-session when sampler is "
+               "running");
+
+    EntryGetter e(*aReader);
+
+    Maybe<double> currentPauseStartTime;
+    Maybe<double> currentCollectionStartTime;
+
+    while (e.Has()) {
+      if (e.Get().IsPause()) {
+        currentPauseStartTime = Some(e.Get().GetDouble());
+      } else if (e.Get().IsResume()) {
+        AddPausedRange(aWriter, "profiler-paused", currentPauseStartTime,
+                       Some(e.Get().GetDouble()));
+        currentPauseStartTime = Nothing();
+      } else if (e.Get().IsCollectionStart()) {
+        currentCollectionStartTime = Some(e.Get().GetDouble());
+      } else if (e.Get().IsCollectionEnd()) {
+        AddPausedRange(aWriter, "collecting", currentCollectionStartTime,
+                       Some(e.Get().GetDouble()));
+        currentCollectionStartTime = Nothing();
+      }
+      e.Next();
+    }
+
+    if (currentPauseStartTime) {
+      AddPausedRange(aWriter, "profiler-paused", currentPauseStartTime,
+                     Nothing());
+    }
+    if (currentCollectionStartTime) {
+      AddPausedRange(aWriter, "collecting", currentCollectionStartTime,
+                     Nothing());
+    }
+  });
+}
+
+bool ProfileBuffer::DuplicateLastSample(BaseProfilerThreadId aThreadId,
+                                        const TimeStamp& aProcessStartTime,
+                                        Maybe<uint64_t>& aLastSample) {
+  if (!aLastSample) {
+    return false;
+  }
+
+  ProfileChunkedBuffer tempBuffer(
+      ProfileChunkedBuffer::ThreadSafety::WithoutMutex, WorkerChunkManager());
+
+  auto retrieveWorkerChunk = MakeScopeExit(
+      [&]() { WorkerChunkManager().Reset(tempBuffer.GetAllChunks()); });
+
+  const bool ok = mEntries.Read([&](ProfileChunkedBuffer::Reader* aReader) {
+    MOZ_ASSERT(aReader,
+               "ProfileChunkedBuffer cannot be out-of-session when sampler is "
+               "running");
+
+    EntryGetter e(*aReader, *aLastSample);
+
+    if (e.CurPos() != *aLastSample) {
+      // The last sample is no longer within the buffer range, so we cannot
+      // use it. Reset the stored buffer position to Nothing().
+      aLastSample.reset();
+      return false;
+    }
+
+    MOZ_RELEASE_ASSERT(e.Has() && e.Get().IsThreadId() &&
+                       e.Get().GetThreadId() == aThreadId);
+
+    e.Next();
+
+    // Go through the whole entry and duplicate it, until we find the next
+    // one.
+    while (e.Has()) {
+      switch (e.Get().GetKind()) {
+        case ProfileBufferEntry::Kind::Pause:
+        case ProfileBufferEntry::Kind::Resume:
+        case ProfileBufferEntry::Kind::PauseSampling:
+        case ProfileBufferEntry::Kind::ResumeSampling:
+        case ProfileBufferEntry::Kind::CollectionStart:
+        case ProfileBufferEntry::Kind::CollectionEnd:
+        case ProfileBufferEntry::Kind::ThreadId:
+          // We're done.
+          return true;
+        case ProfileBufferEntry::Kind::Time:
+          // Copy with new time
+          AddEntry(
+              tempBuffer,
+              ProfileBufferEntry::Time(
+                  (TimeStamp::Now() - aProcessStartTime).ToMilliseconds()));
+          break;
+        case ProfileBufferEntry::Kind::CounterKey:
+        case ProfileBufferEntry::Kind::Number:
+        case ProfileBufferEntry::Kind::Count:
+        case ProfileBufferEntry::Kind::Responsiveness:
+          // Don't copy anything not part of a thread's stack sample
+          break;
+        case ProfileBufferEntry::Kind::CounterId:
+          // CounterId is normally followed by Time - if so, we'd like
+          // to skip it.  If we duplicate Time, it won't hurt anything, just
+          // waste buffer space (and this can happen if the CounterId has
+          // fallen off the end of the buffer, but Time (and Number/Count)
+          // are still in the buffer).
+          e.Next();
+          if (e.Has() && e.Get().GetKind() != ProfileBufferEntry::Kind::Time) {
+            // this would only happen if there was an invalid sequence
+            // in the buffer.  Don't skip it.
+            continue;
+          }
+          // we've skipped Time
+          break;
+        case ProfileBufferEntry::Kind::ProfilerOverheadTime:
+          // ProfilerOverheadTime is normally followed by
+          // ProfilerOverheadDuration*4 - if so, we'd like to skip it. Don't
+          // duplicate, as we are in the middle of a sampling and will soon
+          // capture its own overhead.
+          e.Next();
+          // A missing Time would only happen if there was an invalid
+          // sequence in the buffer. Don't skip unexpected entry.
+          if (e.Has() &&
+              e.Get().GetKind() !=
+                  ProfileBufferEntry::Kind::ProfilerOverheadDuration) {
+            continue;
+          }
+          e.Next();
+          if (e.Has() &&
+              e.Get().GetKind() !=
+                  ProfileBufferEntry::Kind::ProfilerOverheadDuration) {
+            continue;
+          }
+          e.Next();
+          if (e.Has() &&
+              e.Get().GetKind() !=
+                  ProfileBufferEntry::Kind::ProfilerOverheadDuration) {
+            continue;
+          }
+          e.Next();
+          if (e.Has() &&
+              e.Get().GetKind() !=
+                  ProfileBufferEntry::Kind::ProfilerOverheadDuration) {
+            continue;
+          }
+          // we've skipped ProfilerOverheadTime and
+          // ProfilerOverheadDuration*4.
+          break;
+        default: {
+          // Copy anything else we don't know about.
+          AddEntry(tempBuffer, e.Get());
+          break;
+        }
+      }
+      e.Next();
+    }
+    return true;
+  });
+
+  if (!ok) {
+    return false;
+  }
+
+  // If the buffer was big enough, there won't be any cleared blocks.
+  if (tempBuffer.GetState().mClearedBlockCount != 0) {
+    // No need to try to read stack again as it won't fit. Reset the stored
+    // buffer position to Nothing().
+    aLastSample.reset();
+    return false;
+  }
+
+  aLastSample = Some(AddThreadIdEntry(aThreadId));
+
+  tempBuffer.Read([&](ProfileChunkedBuffer::Reader* aReader) {
+    MOZ_ASSERT(aReader, "tempBuffer cannot be out-of-session");
+
+    EntryGetter e(*aReader);
+
+    while (e.Has()) {
+      AddEntry(e.Get());
+      e.Next();
+    }
+  });
+
+  return true;
+}
+
+void ProfileBuffer::DiscardSamplesBeforeTime(double aTime) {
+  // This function does nothing!
+  // The duration limit will be removed from Firefox, see bug 1632365.
+  Unused << aTime;
+}
+
+// END ProfileBuffer
+////////////////////////////////////////////////////////////////////////
+
+}  // namespace baseprofiler
+}  // namespace mozilla